Effects of trisodium citrate on morphology of β-NaGd1−x Yb x F4:Er3+ nanocrystals: role of Yb3+ concentration

In solvothermal synthesis of Er 3+ -doped NaGd 1− x Yb x F 4 nanocrystals, morphology of the nanoparticles is dependent on the molar ratio of the chelating agent (e.g., trisodium citrate) to the lanthanide ions. Usually, addition of chelating agent is reported to be effective only when the molar ratio is below a critical value usually around 1 for the preparation of Er 3+ - and Yb 3+ -doped NaYF 4 nanocrystals. In this work, we found that, by increasing the Yb 3+ concentration, the critical molar ratio can be up to more than 4 in the preparation of Er 3+ -doped NaGd 1− x Yb x F 4 nanocrystals, which provides a new route to control the nanoparticles’ size. The result is particularly useful for the synthesis of small Er 3+ -doped NaGd 1− x Yb x F 4 nanocrystals with high concentration of the Yb 3+ dopant. We also investigated mechanisms on the formation of such particles ( β -NaGd 0.3 Yb 0.7 F 4 :Er 3+ ) with large size in a sub-micrometer scale, i.e., the aggregation-induced crystal growth. In addition, upconversion luminescence properties of the powdery samples were studied, which is shown to be dependent both on size of the nanoparticles and on dosage of the trisodium citrate.